Can I use my heated propagator in summer?

I think that you probably already know the answer to you question, so really it is just a question of confirmation and explanation as to why. Wonderful that you have bought a heated propogator, and when used to bring the seeds to a stage of germination when the temperature is low can be a great bonus. I f you put seeds into a heated propogater when the temperature is high it is likely that the propogator will not work if it is the type that has a thermostat. If it just has a very low wattage heating element with no thermostat it will almost certainly heat up to an unsatisfactory temperature. The seeds will possibly start to germinate, but will then be prone to rotting or even racing away and becoming 'leggy'. If the temperature is warm enough, the seeds will germinate in either the ground or a container without artificial help. To promote rapid healthy growth plants also require light and food. This is best acheived naturally when the conditions are right. Good luck. Try experimenting. Gardening can be great fun and very rewarding.